The invention relates to an ultrasonic fluid meter including ultrasound transducers defining between them an ultrasonic measurement path and emitting and receiving ultrasound waves into the fluid along said measurement path at at least one ultrasound frequency.
It has been known for many years that the speed of a flowing fluid can be measured by emitting ultrasound waves into the fluid from ultrasound transducers both in the flow direction of said fluid and in the opposite direction, and by measuring the respective propagation times of the waves emitted in both directions.
Starting from a measurement of the speed of the fluid, it is easy to determine its flow rate and also the volume of fluid that has flowed over a given length of time.
Nevertheless, in such fluid meters, the Applicant has observed that xe2x80x9cparasiticxe2x80x9d ultrasound waves propagate and disturb reception at one of the transducers of the ultrasound waves emitted by the other transducer along the measurement path.
Two different types of parasitic ultrasound wave can be mentioned: ultrasound waves generated by a source outside the fluid meter; and ultrasound waves emitted by the transducers themselves.
The first type is encountered, for example, when a pressure regulator is installed upstream from a gas meter.
Pressure regulators are used, for example, to reduce pressure of a gas from several bars down to about 20 mbars upstream from ultrasonic gas meters. Unfortunately, the pressure drop in the regulator is a source of a considerable amount of noise and it has been observed that such a pressure drop can give rise to parasitic ultrasound waves of great pressure amplitude and of frequency(ies) corresponding to the frequency(ies) of the ultrasound transducers of the meter.
These parasitic ultrasound waves are conveyed by the fluid flow to the ultrasound transducers. This gives rise to considerable errors in measurement that are completely unacceptable.
The first type of parastic ultrasound can also be encountered in ultrasonic liquid meters which are placed downstream from a sharp reduction of flow section that can give rise to a phenomenon known as xe2x80x9ccavitationxe2x80x9d with bubbles appearing in the liquid at a frequency close to that used by the ultrasound transducers.
The second type of parasitic ultrasound wave corresponds to the case where the ultrasonic measurement path defined between the ultrasound transducers lies within a duct (tube, . . . ) conveying the fluid whose flow rate is to be determined and where the duct is made of a material that is not sufficiently stiff to prevent acoustic coupling between the fluid medium and said material.
This can be the case, for example, when the duct is made of metal (steel, . . . ) and the fluid medium is water, or indeed when the duct is made of plastic and the fluid is a gas.
Under such circumstances, when ultrasound waves are emitted from one transducer towards the other inside the measurement duct, a portion of these waves, referred to as xe2x80x9cparasiticxe2x80x9d ultrasound waves, propagate through the material constituting said measurement duct and reach the other transducer either before or together with the ultrasound waves propagating through the fluid medium.
It thus becomes very difficult to distinguish amongst the ultrasound waves received from the other transducer between those which have indeed propagated in the fluid medium and those which have propagated in the medium constituting the measurement duct.
Document EP-A- 0 457 999 describes an ultrasound flow rate meter comprising a duct, in which the fluid whose flow rate is to be determined flows, and two transducers situated outside the duct. The ultrasound waves generated or received by the transducers are respectively transmitted to the fluid or received from the fluid by means of disks associated with the transducers and wall portions situated facing the transducers. The device described includes dampers or groove/projection pairs whose object is to uncouple the wall portions from each other situated facing the transducers. Firstly, such a device uses a measuring principle aiming to cause the wall of the tube where the liquid flows to resonate, and secondly, it does not resolve the problem of the parasitic ultrasound waves of the first type.
Document FR-A-2 357 869 discloses means for attenuating soundwaves generated outside an ultrasound fluid meter and which are implemented in the form of a sleeve of acoustically insulating material placed in the fluid inlet fitting of the meter. Unfortunately, such attenuation means are insufficient and, in addition, they are incapable of attenuating parasitic ultrasound waves of the second type.
Document EP-A-0 048 791 discloses a device for eliminating ultrasound waves emitted by the transducers outside the measurement tube. Unfortunately, such a device does not enable ultrasound waves propagating in the wall of the measurement tube to be attenuated, nor does it attenuate parasitic ultrasound waves of the first type.
The present invention therefore seeks to remedy this problem by attenuating in simple and effective manner parasitic ultrasound waves propagating in an ultrasonic fluid meter and disturbing reception by one of the transducers of ultrasound waves emitted by the other transducer along the measurement path.
The present invention thus provides an ultrasonic fluid meter comprising ultrasound transducers defining between them an ultrasonic measurement path and emitting and receiving ultrasound waves into the fluid along said measurement path at at least one ultrasound frequency, and means for attenuating xe2x80x9cparasiticxe2x80x9d ultrasound waves of wavelength xcex which disturb reception by one of the transducers of ultrasound waves emitted by the other transducer, wherein the attenuation means are constituted by at least one passage in which said parasitic waves propagate in a main direction corresponding to a xe2x80x9clongitudinalxe2x80x9d dimension a of said passage, said passage having a transverse direction b perpendicular to the dimension a and much smaller than the wavelength xcex of the parasitic waves in the propagation medium, said passage comprising a plurality of consecutive passage portions each having a part that presents a reduction in transverse propagation section along the dimension b of the passage, with the longitudinal dimension of each passage portion being substantially equal to xcex/2.
Thus, the parasitic ultrasound waves which propagate in the propagation medium mainly along the longitudinal dimension of the passage encounter on their path reductions in propagation section alternating with xe2x80x9cnormalxe2x80x9d propagation sections, thus creating an acoustic impedance discontinuity in the medium, which reflects a portion of the energy contained in these waves, thereby attenuating the amplitude of these waves. Parasitic ultrasound waves that do not propagate along the longitudinal dimension of the passage but which nevertheless encounter small propagation sections on their path are also attenuated.
According to a characteristic of the invention, the passage is defined by at least two longitudinal surfaces facing each other and spaced apart along the dimension b, and on which at least one of them has a plurality of mutually parallel consecutive grooves formed in alternation with projections, each passage portion having a pair constituted by a groove and a projection.
By way of example, each groove has a V-shaped profile or a U-shaped profile thereby causing said surface to be crenellated.
In a variant embodiment, the parasitic waves are of wavelength that varies within a determined range, and the longitudinal dimension of the passage portions varies in increasing or decreasing manner to cover the determined range of wavelengths.
In a first aspect of the invention, the fluid meter comprises an enclosure provided with a fluid inlet orifice and a fluid outlet orifice, a measurement block fitted with ultrasound transducers and provided with at least two openings respectively enabling the fluid to reach the ultrasonic measurement path and to leave it, the passage(s) in which the parasitic ultrasound waves propagate being formed between the measurement path and at least one of the fluid inlet and outlet orifices.
In a first embodiment of the invention, the measurement block is disposed inside said enclosure in such a manner as to form between them the passage(s) in which the parasitic waves propagate, and along which the fluid flows prior to entering the measurement block or after leaving it.
For example, the surface on which the grooves are formed is the surface of the measurement block.
In a second embodiment, the measurement block includes the passage(s) enabling the parasitic ultrasound waves to be attenuated and formed between at least one of said openings and said measurement path, said passage(s) also serving to convey the fluid.
According to other characteristics of the invention:
the measurement path is formed within a measurement duct;
the measurement duct is at least partially disposed in a housing of the measurement block;
the passage(s) is (are) formed between the walls of the housing and the measurement duct;
the passage(s) is (are) formed around the measurement duct;
the passage(s) is (are) formed on one side only of the measurement duct;
the passages are formed on either side of the measurement duct;
the measurement block includes a xe2x80x9cseparatorxe2x80x9d wall on its side where the passage(s) is (are) formed to separate the measurement duct from the passage(s);
the measurement block includes another wall which is disposed facing the separator wall in such a manner that the facing surfaces of these two walls define the passage(s);
the other wall is a separate part fitted to the measurement block;
the groove/projection pairs are formed on the outside surface of the measurement duct;
the passage(s) is (are) formed along at least a portion of the measurement path; and
the passage(s) is (are) disposed inside the measurement duct.
In a second aspect of the invention, the fluid meter comprises a measurement duct constituting at least a portion of the ultrasound measurement path and presenting at least one peripheral wall corresponding to the passage in which the parasitic ultrasound waves propagate.
The surface on which the grooves are formed is the outside surface of the measurement duct, the reduction of the propagation section in each passage portion being located at each groove in said wall.
For example, the measurement duct is a tube.
The grooves are annular in shape and are disposed along the tube.
In a variant, a groove of helical shape is formed in the outside surface of the measurement tube.
Advantageously, the measurement tube can be screwed into a housing provided in the measurement block.